Conventionally, a lead storage battery for automobile is employed as a power source in actuating a starter motor for starting the engine, as a power source for lighting or ignition, or as a power source for various kinds of motors which may be mounted in an amount of 100 or more in the case of a high-class car.
However, since the lead storage battery for automobile is employed in such a manner that electric power is always supplied to the lead storage battery through the driving of a generator by the engine of the automobile except the moment when the starter is actuated for starting the automobile, the lead storage battery for the automobile is not discharged so deeply up to date. Rather, since the lead storage battery is always placed into a state of over-charge in most cases due to the charging by the automobile generator, the lead storage battery is conventionally required to be highly resistive to the over-charging. Additionally, it is required that the lead storage battery is constructed such that the decrease of electrolyte due to the generation of gas during over-charging can be inhibited, thus not requiring the replenishment of water and rendering the battery excellent in being maintenance free. In view of this, the alloy now employed for the positive electrode is made of Pb—Ca type alloy instead of Pb—Sb type alloy.
However, since there are strong demands to improve the fuel consumption and to minimize the noxious exhaust gas in automobile in recent years, the conditions for operating the lead storage battery for automobile have been greatly changed.
One example of such operating conditions is the charge-control, i.e., suppression of charging to a lead storage battery. Conventionally, the charging of lead storage battery for automobile has been performed through the operation of generator by the engine as in the case of feeding electric power to other electrical equipments. Therefore, the lead storage battery is always brought into a state of over-charging, thereby naturally deteriorating the fuel consumption. In view of this, the charge-control of lead storage battery is now actually executed, thus improving the fuel consumption and minimizing the obnoxious exhaust gas.
Further, in view of improving the fuel consumption through prevention of over-charging of lead storage battery, it is also proposed to detect the state where the charging of lead storage battery is required and to perform the charging of lead storage battery only when such a state is recognized, thereby avoiding redundant charging and improving the fuel consumption.
However, if the charging of lead storage battery is to be performed only when the charging is found required as a result of the detection of the aforementioned partial state of charge (PSOC) or the fuel consumption efficiency, the opportunity of performing the charging of lead storage battery would be restricted, so that the lead storage battery would be always used in a PSOC. However, if the lead storage battery is used in such a partial state of charge, the lead storage battery, especially the lead storage battery which is low in charging efficiency, would be brought into a state of chronic insufficient-charging on the contrary. In that case, it may be required to frequently perform so-called refresh charge wherein the state of charge (hereinafter referred to as SOC) of the lead storage battery is increased up to 100% in order to overcome the aforementioned state of chronic insufficient-charging. As a result, the fuel consumption would be deteriorated on the contrary.
Another example of such operating conditions in the employment of lead storage battery is an operation accompanying so-called idling-stop where the engine is stopped during the stoppage time due to a stop signal, etc. During this idling-stop, the feed of power from the generator is also suspended due to the stop of engine. As a result, the feed of power during this time will be covered by the discharging from the lead storage battery, thus increasing the opportunity of discharging of lead storage battery as compared with the conventional operation of the battery, thus also resulting in the use of the battery in a PSOC. In this case also, so-called refresh charge of lead storage battery is required to be frequently performed, resulting in the deterioration of fuel consumption.
The state of charge of the lead storage battery in this PSOC is generally confined within the range of more than 70% to less than 100%. Because, in the case of automobile where the start of engine is guaranteed by the use of only one lead storage battery in principle, there are possibilities of raising a problem in the start-up of engine if the SOC is not more than 70%. Therefore, the lower limit of the SOC is generally set to more than 70%.
On the other hand, 100% in the state of charging cannot be achieved unless the lead storage battery is always charged into a state of over-charging. However, this over-charging would become a cause for deteriorating the fuel consumption as described above. Therefore, the upper limit of the SOC is generally set to less than 100%.
Further, in the case of a hybrid system (HVS) where the brake-regenerating electric power is temporarily stored in a lead storage battery and then quickly discharged on the occasion of assisting the acceleration, even if the lead storage battery is to be operated in a partial state of charge as described above, the lead storage battery is usually operated by setting the upper limit of state of charge to not more than 70% in order to secure a high charging efficiency. As for the prior art concerning the lead storage battery having a hybrid function where regenerated electric power is utilized in the charge and discharge of lead storage battery, it is described in JP Laid-open Patent Publication (Kokai) No. 2003-36882 and JP Laid-open Patent Publication (Kokai) No. 2003-51334. According to these publications, the SOC is always confined to the range of 50-70% in order to perform quick charging at high-efficiencies. In this case however, a battery for starting engine is separately mounted so as to obviate any problem in a low temperature start-up of engine.
When the charge-control or idling-stop is executed under this condition using the conventional lead storage battery which is designed attaching importance to the resistance to the corrosion of positive grid or gross on the occasion of over-charging, it is impossible to obtain a sufficient charging efficiency in spite of the fact that the opportunity of charging is not so many. Because of this, the conventional lead storage battery is liable to be brought into a state of chronic insufficient-charging. If this problem is to be overcome, it is required to frequently perform the refresh charge, thus making it impossible to sufficiently contribute to the improvement of fuel consumption which the charge-control is inherently aimed at.
Further, since the conventional lead storage battery is brought into a state of chronic insufficient-charging when the lead storage battery is kept in a partial state of charge, not only the surface of negative electrode but also the surface of positive electrode is suffered from a phenomenon of sulfation where lead sulfate is accumulated, thus raising the problem that the life of lead storage battery is considerably shortened.
In connection with this problem, there are proposed a flooded lead storage battery and a sealed lead battery, wherein about 0.1 mol/L of sulfate of alkaline metal (such as sodium) or sodium alum which is a double salt consisting of sodium and aluminum is added to the electrolyte for the purpose of preventing the short-circuit to be caused by the lowering of specific gravity of electrolyte due to a continuous discharging (JP Laid-open Patent Publication (Kokai) No. 8-64226 (1996)).
However, it has been made clear as a result of extensive studies made by the present inventor that even though the aforementioned object can be achieved in the case of the conventional lead storage battery to be employed in full charge state, there is a problem in the case of the lead storage battery which is designed to be employed in a PSOC that the charging efficiency thereof is caused to considerably deteriorate due to the influence of sodium ion and that this bad influence is far greater than the aforementioned object.
As for the means to overcome the sulfation of negative electrode of lead storage battery, there is known an idea of adding carbon to the negative electrode at a larger quantity than ordinary employed (Journal. Power Sources vol. 59 (1996), 153-157). Although nothing is disclosed about the adding quantity of carbon in this prior publication, it is described therein that the carbon added in this manner is enabled to enter into interstices of lead sulfate to thereby create a conductive pass. Therefore, various kinds of tests were performed by the present inventor, wherein a wide range in quantity of carbon was tried in these tests. As a result, it was confirmed that the effects of elongating the life of the lead storage battery are limited under the conditions of charge-control or idling-stop, and that it was difficult, in industrial viewpoint, to put the idea into practical use under the conditions of charge-control or idling-stop.
There is also known a prior art wherein an organic acid such as polyacrylic acid or ester is added to an electrolyte (JP Laid-open Patent Publication (Kokai) No. 2001-313064). However, this prior art is defective in that since the grid is caused to corrode, it is not suited for practical use. Furthermore, there is also known a prior art wherein titanium, aluminum or potassium is added to a gel-like electrolyte to improve the low temperature start-up performance (JP Laid-open Patent Publication (Kokai) No. 60-211777 (1985)). However, the technique disclosed in this prior art is liable to deteriorate the electric conductivity of electrolyte, failing to contribute to the alleged improvement. Further, there is also known a prior art wherein selenium and an organic acid are added to an electrolyte so as to suppress the generation of hydrogen from the negative electrode and to promote the reduction of oxygen (JP Laid-open Patent Publication (Kokai) No. 64-38970 (1989)). According to this prior art however, the quantity of selenium to be added is as large as 100-1000 ppm, causing the selenium to precipitate in the electrolyte, thus badly affecting the lead storage battery on the contrary.
Another reason for shortening the life of lead storage battery can be ascribed to the fact that due to the demand for the development of maintenance-free lead storage battery, the material for the positive substrate of lead storage battery is changed to a Pb—Ca type material from a Pb—Sb type material. In the case of the Pb—Sb alloy that has been conventionally employed, pentavalent antimony ion that has been generated from the oxidization of the substrate is enabled to act on an active material so as to enhance the adhesion of the grid-active material interface, thus gelating part of the active material to strengthen the bonding among the active material. As a result, even if deep charging/discharging is repeated, it is possible to inhibit the peeling between the grid and the active material or the softening of the active material.
In the case of the Pb—Ca type alloy however, the aforementioned effects that can be achieved by antimony are rather weakened. Therefore, when deep charging/discharging is repeated, the active material is caused to peel away from the grid at an early stage and the bonding among the active material is deteriorated and hence the active material is softened, thus shortening the life of battery.
There has been proposed by the present inventor an alloy for the substrate of lead storage battery (JP Laid-open Patent Publication (Kokai) No. 2003-306733), which makes it possible to enhance the corrosion resistance and mechanical strength of electrode plate, the alloy comprising 0.02-0.05 wt % of Ca, 0.4-2.5 wt % of Sn, 0.005-0.04 wt % of Al, and 0.002-0.014 wt % of Ba. This alloy may further include at least one kind of element selected from the group consisting of 0.005-0.07 wt % of Ag, 0.01-0.10 wt % of Bi, and 0.001-0.05 wt % of Ta.
However, it has been found that even in the case of this high-corrosion resistance alloy for substrate, features such as the adhesion between positive substrate and the active substrate and the bonding property among the active material are rather inferior as compared with the conventional Pb—Ca type alloy, thus raising problems in these respects.
It is considered that calcium ion is inherently provided with a function of enhancing the adhesion between the grid and the active material, or the adhesion among the active material (Journal. Power Sources vol. 64 (1997), 51-56). Certainly, it has been recognized that the calcium ion that has been eluted from the alloy substrate containing 0.06-0.1 wt % of calcium has such a function.
On the other hand, it is considered that the corrosion resistance of the alloy tends to increase as the content of Ca in the alloy is decreased. In the case of the substrate alloy disclosed in the aforementioned JP Laid-open Patent Publication (Kokai) No. 2003-306733 for instance, the corrosion resistance of the alloy was considerably improved as the content of Ca was less than 0.05 wt %. On the contrary however, it has been found that, in this alloy, the feeding of calcium ion eluted from the alloy to the active material is caused to decrease, thus making it difficult to enhance the adhesion among the active material.
There is also known a technique which is aimed at overcoming the aforementioned problem, wherein a layer comprising antimony is deposited on the surface of positive substrate, or an antimony compound is added to the positive substrate, thereby obtaining the same effects as obtainable by the employment of Pb—Sb type alloy (JP Laid-open Patent Publications (Kokai) No. 49-71429 (1974); No. 53-75444; and No. 63-148556).
Additionally, there are known various techniques aimed at maintaining the same degree of bonding strength of active material as obtainable by the employment of antimony, such prior art including a technique wherein the surface layer of electrode substrate is constituted by a lead alloy layer containing at least one element selected from alkaline metal and alkaline earth metal (WO-01/04976-A1); a technique wherein tin dioxide and calcium sulfate are incorporated into the active material layer (JP Laid-open Patent Publications (Kokai) No. 9-289020 (1997)); and a technique wherein 0.5-5 wt % (as reduced to metal tin and based on positive active material) of metal tin or a tin compound is incorporated into the positive active material and, at the same time, the density of positive active material is confined to 3.8-5.0 g/cc (JP Laid-open Patent Publications (Kokai) No. 10-188963 (1998)). All of these prior arts are certainly effective under the conditions where the conventional lead storage battery is to be employed.
However, under the conditions of use where a lead storage battery is subjected to charge-control or idling-stop, since the charging/discharging is repeated in a PSOC for a long period of time, the aforementioned countermeasures are insufficient in coping with problems of the shedding or softening of active material.
Moreover, in addition to the requirements of the lead storage battery to improve the fuel consumption and reduce the exhaust gas, it is further required to enhance the start-up performance (discharge property) and positive active material, thus making it possible to decrease the weight of battery. In view of enhancing the positive active material of the lead storage battery, it has been tried to improve the dispersion of electrolyte. For this purpose, a method of decreasing the density of positive active material is generally employed.
However, when a Pb—Ca type alloy is employed as a positive grid, the active material is liable to be peeled off from the grid due to charging/discharging and additionally the active material is caused to soften and shed, thus shortening the life of battery considerably. There has been proposed a method of incorporating graphite into the positive active material in order to overcome the aforementioned problems.
In this case, graphite is caused to expand as sulfate ion intercalates into interstices due to the injection of an electrolyte, thus creating voids in the active material, these voids in the active material being further increased due to the dissipation through oxidation during charging. However, the expansion of graphite invites at the same time the destruction of active material, thus also resulting in the shortened life of battery. Because of this, the aforementioned technique is applicable only to a seal type lead battery where a group of plates are strongly compressed. However, even in this case, the battery jar may be caused to deform or destroyed due to the expanded positive electrode.
With a view to overcome the aforementioned problems, there has been proposed a technique wherein lead powder, red lead, fibrous resin, expanded graphite and dilute sulfuric acid are kneaded together under a reduced pressure to form a paste, which is employed for fabricating the active material (JP Laid-open Patent Publication (Kokai) No. 2004-55309). Although all of these prior arts are certainly effective under the conditions where the lead storage battery is conventionally employed, the aforementioned prior arts are not sufficiently effective under the conditions of use where a lead storage battery is subjected to charge-control or idling-stop, since the charging/discharging is repeated in a PSOC for a long period of time.